The present invention generally relates to power system control, and more specifically, to an apparatus and methods for providing a voltage adjustment for single-phase voltage regulator operation in a three-phase power system.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads via a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, distribution lines, buses and transformers, power transmission lines, distribution lines, buses and capacitors, to name a few. As a result, power systems typically include a number of regulators having associated control devices, and many protective devices having associated protective schemes to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
In general, protective devices and their associated protective schemes act to isolate a power system element(s) (e.g., a generator, transformers, buses, motors, etc.) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s). Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosures.
Regulators and their associated control devices are utilized to regulate the voltage level in the power system. For example, a number of single-phase step voltage regulators may be coupled to the various transmission, sub-transmission and distribution lines (collectively, “distribution lines”) to enable voltage regulation of the distribution line to, for example 13 kV±10 percent, during a wide range of load conditions (e.g., a plant coming on-line). Such voltage regulators are often located adjacent to a step-down power transformer and generally include an autotransformer having a single winding (e.g., a series winding), which is tapped at some tap position along the winding to provide a desired voltage level.
A typical step voltage regulator may have a 100 percent exciting winding in shunt with the distribution line on the source side, and operate to maintain a voltage on the load side of the distribution line. The voltage is maintained within a desired voltage bandwidth by means of a 10 percent tapped buck/boost winding connected in series with the distribution line. The series winding has taps connected to stationary contacts of a tap changer dial switch, where the tap changer dial switch includes a pair of rotatable selector contacts driven by a reversible motor into sequential engagement with the pairs of contacts. For example, the tap changer dial switch may enable a capability to change the effective turns ratio from input to output ±10 percent in 32 steps of ⅝ percent each or 0.75 V. A voltage control device, operatively coupled to the voltage regulator may also be included to select the proper tap position or tap for voltage regulator operation based on power system conditions.
Voltage regulators operate via a comparison of an actual measured voltage (i.e., a secondary distribution line voltage) to some internal fixed reference voltage, or center-band voltage. A voltage difference is amplified and used to control operation of the voltage regulator via the voltage control device. Thus, if the measured voltage is too high or in a first out of band (OOB) area above an in-band area, the voltage regulator is directed by the voltage control device to execute a tap change to yield a lower voltage. If the measured voltage is too low, or in a second OOB area below the in-band area, the voltage regulator is directed by the voltage control device to execute a tap change (e.g., a one tap position change) to yield a higher voltage.
Because currents resulting from a fault can easily exceed 10,000 amperes (amps) and because the voltage control device is designed to utilize currents and voltages much less than those of the distribution lines, the currents and voltages are stepped-down via current and voltage transformers, respectively. As is known, the three-phase current and voltages are commonly referred to as the primary current and voltages, while the stepped-down current and voltages are referred to as the secondary current and voltages, respectively. The stepped-down secondary current and voltages are digitized and utilized to determine corresponding phasors representative of the primary current and voltages. The phasors may then used while executing the voltage control logic scheme of the voltage control device to determine whether a tap change is required by the voltage regulator (discussed below).
In some cases, the voltage control device may cause a tap limit to be reached; that is, due to lower measured voltages over time, the voltage control device causes the voltage regulator to continue to change taps to increase the voltage delivered to the load until there are no more available taps. As a result, further decreases in the load voltage can not be addressed via a tap change.
The problem of the tap limit may be addressed by adjusting the center-band voltage to a lower voltage via subtracting a percentage of the center-band voltage setting from the center-band voltage setting, thereby effectively lowering the reference voltage used by the voltage control device. For example, using Kirchoff's law, V=I*Z, if the center-band voltage setting is decreased from 120 V to 118 V, a constant impedance load will draw less current, thereby reducing the overall system load. Although lowering the center-band voltage setting is effective when the load is predominantly of the constant impedance type, it does not always result in a smooth “system voltage” profile.